Hybrid circuits can be fabricated on an insulating substrate comprised of an alumina or similar ceramic. The hybrid circuit combines any number of thick-or thin-film components, semiconductor devices and discrete parts such as capacitors. Hybrid circuits are typically employed in high frequency and microwave applications, although low frequency applications can also be implemented. In many cases, an insulated lid is attached to the substrate encapsulating the circuitry. In high power applications, it is often desirable to include a heat sink. Typically, a heat sink is directly attached to the inactive side of the hybrid with thermal epoxy.
In the design and manufacture of electronic devices that are operated at high frequencies, the electrical interconnection between circuits is of great concern. An interconnection must be treated as a controlled-impedance transmission line when the interconnection dimension becomes a significant fraction of the signal wavelength (typically 1/10 to 1/8 of the wavelength is considered significant). At some high frequencies, and microwave frequencies (i.e., frequencies greater than 1 Ghz) nearly all circuit interconnections meet this criterion and, consequently, must be designed as controlled-impedance transmission lines. Interconnections which do not match the characteristic impedance of the circuit, as well as those that are not field matched, result in signal reflections and, thus, loss of power transmission and signal integrity.
Typically, a high frequency circuit, which is attached to the substrate, will have conductive pads that are etched near the perimeter of the circuit. These pads are electrically connected via wire bonding to conductive leads that are etched or printed onto the substrate. In order to provide a controllable interconnection between the pads and leads, a resistor often terminates each lead. A problem with this technique is that each terminating resistor is printed as a single instance. This conventional technique increases the chance for process errors and non-uniform terminating resistors since multiple screen prints are required to terminate multiple leads. Furthermore, the terminating resistor contains slag as a result of laser trimming to precise value. This slag additionally ends up on the conductive leads which impedes the wire bond. Still further, conventionally printed and laser trimmed resistors do not facilitate dense packaging due to their typical thickness (i.e., approximately 10-20 mils).